As is well known, the number of transistors on integrated circuits approximately doubles every two years, in accordance with Moore's Law. However, the continuation of CMOS scaling beyond the 12 nm technology node will only be possible if device engineers replace the traditional silicon channel of e.g. a transistor by more powerful semiconductor materials. High carrier mobility materials such as germanium and/or compound semiconductor materials have now matured to an industrial development level, but even these material reach their limits in view of very small devices.
A next generation of materials are two-dimensional materials such as graphene, which is in fact a two-dimensional sheet of carbon atoms arranged in hexagonal rings. Graphene offers extremely high carrier mobilities but has basically no bandgap. Measures to open up the bandgap (‘chemical functionalization’) are known but deteriorate at the same time its mobility, hence graphene is losing much of its attraction as alternative MOS channel material.
Other two-dimensional materials got in the picture, such as transition metal dichalcogenides (TMDCs), denoted by the formula MX2, where M denotes a transition metal such as e.g. W or Mo, and X denotes a chalcogen, i.e. a non-metal of the oxygen group, such as e.g. S, Se or Te. TMDCs have found interest as alternative candidate 2D materials due to the fact that they have a natural finite band gap, in contrast to graphene. Their unique structure moreover results in a highly inert, trap-free basal surface of the single crystals and enables fabrication of FETs with an intrinsically low field-effect threshold and a high mobility of charge carriers, comparable to that in the best single-crystal Si devices. Such TMDC materials, especially for instance MoS2, are known already for a long time as lubricants, and have properties very similar to graphite. In their bulk state, they are composed of a layered material with strong in-plane bonding and weak out-of plane interactions (the layers are only weakly bonded by van der Waals forces), such that individual layers can easily move with respect to each other and in this way reduce friction between moving parts. It is known e.g. from WO 2012/093360 (A1) that the properties of a bulk structure with many layers of such material is quite different from the properties of only a single layer or a double layer of such material, so the question arises how such a single layer or double layer can be produced.
Similar as in graphite, individual layers of MoS2 can be separated by the exfoliation technique (see e.g. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, “Single-layer MoS2 transistors”, Nat. Nanotechnology 2011, 6, 147-151), resulting in flakes with dimensions on the order of 1-10 μm. In contrast to the single atomic graphene layer, MX2 is composed of a single metal layer sandwiched between two chalcogen atomic layers which are each arranged in a 2D hexagonal honeycomb structure, as illustrated in FIG. 1. On the left of FIG. 1 a top view is shown, and on the right of FIG. 1 a side view is shown of a stack consisting of four tri-atomic layers. Hereafter, reference to such a tri-atomic layer structure will be made as a “monolayer” of MX2 material. Two polymorphs of MoS2 are the so-called 2H and 3R symmetries characterized by a different stacking of the monolayers, as illustrated in FIG. 2. Of these, the 2H is the more stable one. When reducing the thickness of MX2 from several layers to one single monolayer, the bandgap evolves from indirect to direct, which makes it also interesting for optical applications in integrated circuits. Over the last few years, many reports emerged showing impressive progress in the development of 2D FETs with quite high mobility and very high Ion/Ioff ratios (see for instance S. Balendhran et al., “Two-Dimensional Molybdenum Trioxide and Dichalcogenides”, Advanced Functional Materials 2013, DOI: 10.1002/adfm.201300125).
It is a major challenge however to produce MX2 layers in relatively large areas and/or with the desired orientation (e.g. substantially parallel to the substrate) so as to make them fit for integration into high-volume semiconductor manufacturing on e.g. 300 or 450 mm semiconductor substrates, e.g. silicon wafers. The best known method for making device-quality MX2 is the exfoliation technique applied on naturally occurring mono-crystals of MX2. This results in tiny flakes of only few micrometer size. Many approaches have been studied to produce samples of layered MX2, but as far as known to the inventors, so far none of them resulted in techniques suitable for manufacturing on industrial scale. Industrial alternatives to exfoliation are deposition methods. These methods can be categorized in vapor phase, liquid phase (electrodeposition, hydrothermal, sol/gel) and solid state techniques. In many cases, uncontrolled multi-layer deposits are obtained which still necessitates mechanical or other exfoliation steps. The liquid phase deposition and solid state reaction approaches did not prove very successful, and do not seem fit for large scale integration. There are mainly two routes of Vapor phase deposition, i.e. physical and chemical vapor deposition (PVD and CVD). Most of the PVD routes (e-beam evaporation, RF and DC sputtering, . . . ) have problems with obtaining flat layered structures and result rather in nanodots or nanotubes, or they are too slow and expensive (e.g. molecular beam epitaxy). Other approaches are based on consecutively depositing the metal or metal oxide (e.g. Mo or MoO3) by evaporation followed by a chalcogenidation reaction in e.g. X or H2X vapor. As these reactions proceed typically at very high temperature, e.g. in the order of 650-1000° C., and/or require long annealing times, e.g. in the order of several hours, they are not very efficient. Moreover, often they do not result in large area flat deposits with the desired horizontal (i.e. parallel to the substrate surface) layering. Amongst the CVD routes, three processes can be considered: chemical vapor transport (CVT), regular CVD based on chemical precursors at temperatures above 600° C., and atomic layer deposition (ALD). For instance T. W. Scharf, D. R. Diercks, B. P. Gorman, S. V. Prasad & M. T. Dugger (2009) “Atomic Layer Deposition of Tungsten Disulphide Solid Lubricant Nanocomposite Coatings on Rolling Element Bearings”, Tribology Transactions, 52:3, 284-292, discusses WS2 deposition for use as lubricant coating on roller element bearings, and a process requiring catalysis by means of diethyl zinc ((C2H5)2Zn) in order to initiate and maintain growth on SiO2, Al2O3, stainless steel, gold or poly-crystalline silicon. However, the Zn which is co-deposited is not desirable for proper MX2 qualities (a.o. crystalline state and electrical characteristics) for semiconductor applications.